Two experiments with obsessive-compulsive patients test the efficacy of modalities of treatment by exposure and response prevention. The first study examines the differential effectiveness of exposure alone or response prevention alone or a combination of the two. Exposure and response prevention combined are highly stressful for patients. If either component alone yields similar results, it will constitute a less stressful treatment alternative and consequently a preferable one. Additionally, this study will provide information about theoretical issues basic to the exposure paradigm. The second study tests the effectiveness of exposure in imagination to ruminations of anticipated disasters in patients who are checkers or washers. Here the differential effect of exposure in imagination or exposure in vivo is studied. This investigation bears on the general issue of the relationship between cognitive, physiological and behavioral responses during habituation and treatment outcome. Variables associated with failure to respond to the optimal behavioral treatment are explored, in an attempt to identify such patients before commencement of treatment and to develop procedures that will enable them to improve. This line of research is expected to contribute to the understanding of fear reduction processes in general. In an additional study we are investigating subjective and physiological responses under two conditions of prolonged exposure: distraction and attention-focusing. Results from this study are expected to provide information about the cognitive variables which facilitate anxiety reduction.